Prior to widespread availability of natural gas, gas for fuel and light was manufactured by the hightemperature carbonization of bituminous coal, resulting in a product called "town gas" or "manufactured gas". These early gasification activities, which date back to the 1800s, resulted in the formation of coal tar residues. These residues were usually either burned on site as a supplemental fuel or land-disposed of near the plant. Villaume, J.F. Hazardous and Toxic Wastes: Technology, Management and Health Effects; Majumdar, S.K., Miller, E.W., Eds.; The Pennsylvania Academy of Science: Philadelphia, PA, 1984: pp 362-375. As a result of such practices, abandoned town gasification sites or disposal sites are now becoming recognized as environmental trouble spots. Due to the widespread use of town gas in the late 19th and early 20th century, there are expected to be many such sites around the country. Consequently, it is necessary to have reliable methods for the analysis of soil samples that could potentially be contaminated with varying quantities of coal tar or other organic wastes.
Most current methods for the measurement of organics in soils and sediments require lengthy solvent extraction. In order to implement remedial measures and to carry out cost-effective site assessments, more rapid and field-adaptable sample extraction methods are desirable. Supercritical fluid extraction (SFE) techniques provide a viable alternative with promising advantages over the current liquid extraction methods. Wright, G.W.; Wright, C.W.; Gale, R.W.; Smith, R.D. Anal. Chem. 1987, 59, 38-44. Schantz, M.M.; Chesler, S.N. J. Chromatoor. 1986, 363, 397-401. Hawthorne, S.B.; Miller, D.J. Anal. Chem. 1987, 59, 1705-1708.
While the SFE technique was originally applied to the analysis of coal tar waste, it is applicable to any type of semivolatile organic compounds that contaminate solid materials. For examples, it has been applied to petroleum tars and PCBs from fly ashed, bottom ashed, sediment, and sludges.
The potential advantages of SFE accrue from the properties of a solvent at temperatures and pressures above its critical point. The liquidlike solvating power and rapid mass-transfer properties of a supercritical fluid provide the potential for more rapid extraction rates and more efficient extraction due to better penetration of the matrix than is feasible with liquids. The properties of a supercritical fluid are intermediate between those of the gas and those of the liquid phases. The compressibility of a supercritical fluid is large just above the critical temperature, and small changes in pressure result in large changes in the density of the fluid. The density of a supercritical fluid is typically 10.sup.2 -10.sup.3 times greater than that of the gas. Molecular interactions increase at these higher densities because of shorter intermolecular distances, and solvating characteristics of the supercritical fluid approach those of a liquid. However, the diffusion coefficients and viscosity of the fluid remain intermediate between those of the gas and liquid phases, thus allowing rapid mass transfer of solutes compared to a liquid. The properties of a supercritical fluid are dependent on the fluid composition, pressure, and temperature. Many fluids have comparatively low critical temperatures that allow extractions to be conducted at relatively mild temperatures. For example, the critical temperature of carbon dioxide is only 31.degree. C. Density or solvating power of a supercritical fluid can be controlled by fluid pressure and/or temperature. In addition, various different fluids or fluid mixtures that exhibit different specific chemical interactions can be used to obtain the desired solvent strength and selectivity.
Although the basis and primary advantage of supercritical fluid extraction is derived from fundamental physical properties, specific instrument design criteria are necessary to fully exploit its potential, particularly for field applications. The present invention was designed specifically for field applications where portability, extraction speed, ease of operation, minimal requirements for ancillary supplies, and samples analysis flexibility (e.g., ability to be analyzed by a variety of analytical approaches) are more significant factors than in laboratory applications. Consistent with this philosophy, the apparatus was designed for use with carbon dioxide, but other pressurized liquids or ambient pressure liquids could also be utilized. The field-portable SFE apparatus of the present invention provides the following operation characteristics:
1. Sample sizes ranging from a few milligrams to several grams (5) can generally be quantitatively extracted in less than thirty minutes. The present device is designed to extract solids (as opposed to liquids) with particles ranging up to 3-4 mm in diameter. Alternative extraction cells could be designed to allow extraction of liquid samples.
2. Wet samples (e.g., containing water) can be extracted directly. It is not necessary to dry a sample before placing it in the extraction cell. The properties of supercritical carbon dioxide and th design of the system allows the water to be removed during extraction and collected (as a separate phase) with the analytes.
3. Rapid extraction is possible due to relatively fast flow rates of the supercritical fluid. Fresh fluid is continuously purged through the sample matrix and depressurized into a collection flask to trap the analytes. Liquid flow rates of several mL (.ltoreq.10) per minute at pressures over 600 bar can be delivered by the pumping system. The maximum operating pressure is limited by the specific extraction cell design. Typical cells allow operation up to 450 bar and cells with "quickconnects" are presently limited to pressures up to 300 bar. Since a reciprocating pump is used, nearly unlimited fluid volumes can be continuously utilized. The pumping system allows control of the fluid pressure (solvating power) enabling selective extraction or fractionation of analytes with different fluid phase solubilities.
4. Extractions can be conducted at temperatures ranging from essentially ambient (although the critical temperature of carbon dioxide is 31.degree. C.) to over 250.degree. C. If extraction cells are used containing polymer seals, it may be necessary to use lower temperatures. Choice of operating temperature is dependent on the nature of the analytes and the matrix. For thermally stable analytes, a high operating temperature may allow more efficient desorption from an adsorptive matrix. For carbon dioxide, maximum operating temperatures of 100.degree.-150.degree. C. are typical. The fluid is preheated prior to entering the extraction cell to provide more homogeneous solvating power through the extraction matrix.
5. Provisions are provided to heat the extraction effluent to higher than the extraction temperature (.ltoreq.400.degree. C.) just prior to analyte collection. The fluid is depressurized by expansion through a restrictor orifice and considerable JouleThomson cooling occurs, particularly at the fast fluid flow rates needed for rapid extraction of larger size samples. This cooling can freeze the orifice closed (with carbon dioxide and/or water) or allow it to become plugged with analytes unless extra heat is added to the system. The actual flow rate of the fluid is governed by the diameter of the restrictor tubing which can range from 50-100 .mu.m i.d.
6. The analytes are collected by bubbling the extraction effluent through an appropriate organic solvent (methylene chloride, etc.) as the fluid is expanded. Studies have shown that near quantitative loss of analytes to the atmosphere can occur through aerosol formation unless the aerosol is disrupted (1). Bubbling the expanded gas through a solvent is a convenient way of disrupting the aerosol. Expansion of a fluid to a gas results in a volume increase of approximately 1000 fold. Consequently, gas flows of over several liters per minute are bubbled through the solvent. To prevent sample and solvent loss, the collection system was designed to efficiently contain the solvent and minimize evaporation. Only 10-20 mL of solvent are needed for each sample. Thus, expensive solvent consumption and lengthy solvent concentration procedures are minimized. A variety of analytical determination methods can then be used for subsequent sample analysis.
7. The apparatus is designed to extract a single sample at a time, but it has tandem sample processing capabilities. This allows near continuous extraction of sequential samples since one sample can be connected or removed from the apparatus while the other one is being extracted.
8. Many of the components of the extraction system are self-cleaning or can be recycled with minimal cleaning. Spent extraction cells are free of extractable organics and can be rinsed with water and air dried.
9. Carbon dioxide cylinders provide a convenient mechanism of supplying a high-purity and non-toxic extraction solvent in the field. Depending on the sample matrix, approximately 5 to 10 samples can be extracted with 1 pound of carbon dioxide. Small aluminum cylinders are available that weigh approximately 50 lbs. and contain 20 lbs. of carbon dioxide. This would service between 100 and 200 extractions.